Chemically compatible, lightweight heat pipe

ABSTRACT

The present invention discloses an apparatus including a magnesium alloy vessel substantially free of aluminum and zinc, with the vessel having a hollow interior cavity containing a working fluid, wherein the improvement includes the formation of a stable, protective layer on the inside wall of the vessel, the layer establishing compatibility with the working fluid and preventing base metal corrosion by the working fluid, the vessel including magnesium in combination with a gettering metal.

BACKGROUND OF THE INVENTION

[0001] A heat pipe is essentially a passive heat transfer device with anextremely high effective thermal conductivity. A two-phase heat transfermechanism results in heat transfer capabilities from one hundred toseveral thousand times that of an equivalent piece of copper. Heat pipesare sealed vacuum vessels that are partially filled with a fluid,typically water in electronic cooling applications, which serves as theheat transfer medium. The heat pipe envelope is typically made ofcylindrical copper tubing, although rectangular cross sections and othermaterials are available. The wall of the envelope is lined with a wickstructure, which generates the capillary force that pulls the condensatefrom the condenser section of the heat pipe back to the evaporatorsection. Since the heat pipe is evacuated and then charged with theworking fluid prior to being sealed, the internal pressure is set by thevapor pressure of the working fluid. As heat is applied to a portion ofthe surface of the heat pipe, the working fluid is vaporized. The vaporat the evaporator section is at a slightly higher temperature andpressure than other areas and creates a pressure gradient that forcesthe vapor to flow to the cooler regions of the heat pipe. As the vaporcondenses on the heat pipe walls, the latent heat of vaporization istransferred to the condenser. The capillary wick then transports thecondensate back to the evaporator section. This is a closed loop processthat continues as long as the heat is applied.

[0002] The orientation and layout of a heat pipe design are important.When the design allows, the heat source should be located below or atthe same elevation as the cooling section for best performance. Thisorientation allows gravity to aid the capillary action, and results in agreater heat carrying capability. If this orientation is unacceptable,then a capillary wick structure such as sintered powder will benecessary. Additionally, heat pipes have the ability to adhere to thephysical constraints of the system, and can be bent around obstructions.

[0003] There is a recurring need for heat pipes having low mass. Therehas been an extended effort to devise a method for using aluminum as theenvelope and wick material. Much of this effort has been to use water asthe preferred working fluid. Previous efforts have been focused ontaking advantage of the fact that aluminum oxide is compatible withwater, even though aluminum metal is not compatible. The programs havenot been successful because of the large difference in thermal expansionbetween aluminum and its oxide. The resulting stresses cause the oxidelayer to crack, often on the first thermal cycle, thereby allowing thewater and aluminum to come into contact, resulting in hydrogengeneration and heat pipe failure.

[0004] The present invention takes advantage of the stabilizing effectsof the “getter” type materials, such as zirconium when added to lightmetals such as magnesium or aluminum. The addition of zirconium to themagnesium provides a more stable oxide and/or nitride, and provides awater-compatible surface. The fact that this alloy is also lighter thanaluminum is an added benefit. The reduced thermal stresses which resultwith this alloy most likely allow the oxide/nitride to maintain itsintegrity.

[0005] Most commercially available magnesium alloys have significantamounts of aluminum, rare earths, and/or zinc as constituents. None ofthese materials are readily compatible with water. Therefore, anadditional objective of the present invention is to specify awater-compatible alloy of magnesium which does not have thesenon-compatible constituents.

SUMMARY OF THE INVENTION

[0006] The present invention is directed to an improvement in heattransfer vessels as used in weight-sensitive applications, e.g., laptopcomputers, these vessels composed of magnesium and substantially free ofaluminum and zinc, these vessels further having a hollow interior cavitycontaining a working fluid. The improvement comprises the formation of astable, protective layer on the inside wall of the vessel, the layerestablishing compatibility with the working fluid, and preventing basemetal corrosion by the working fluid. In a preferred embodiment of thepresent invention, an alloy with no aluminum or zinc, but with 0.5 to 1percent (by weight) zirconium, was used. The zirconium oxide helpsprovide compatibility with water by stabilizing the oxide surface layerin the presence of water, and similarly provides a stabilizing nitridesurface in the presence of ammonia.

DETAILED DESCRIPTION OF THE INVENTION

[0007] The present invention is directed to an improvement in vesselscomposed of magnesium and substantially free of aluminum and zinc, thesevessels having a hollow interior cavity containing a working fluid. Thestable protective layer will be either an oxide or nitride layerdepending on the working fluid. For instance, if ammonia is chosen asthe working fluid, a stable nitride would be formed; in contrast, ifwater is used as the working fluid, a stable oxide would be formed. Thepreferred vessel for purposes of the present invention is a heat pipe,although it is anticipated that other suitable vessels would benefitfrom the purposes of the present invention as well.

[0008] In a further preferred embodiment of the present invention, themagnesium alloy contains 0.1 to 5 percent of a “gettering” metal ormetals e.g., zirconium, titanium, hafnium, yttrium, etc. Amounts ofgettering metal of from about 0.1 to 2 percent are preferred, with anamount of about 1% gettering metal, e.g., zirconium, particularlypreferred.

[0009] A further benefit of the treatment of the present invention canbe the strengthening of the primary metal (e.g., magnesium) withalloying materials in excess of the metal's natural solubility. Thisresults in the dispersal of unalloyed particles in grain boundaries,thereby providing a further strengthening effect.

[0010] In the testing of the present invention, there have been morethan 30 on/off thermal cycles where a Mg/water heat pipe was heated tobetween 100 to 120° C., and then cooled to room temperature. Note that 1to 2 of those cycles are normally enough to cause failure in analuminum/water heat pipe. It has been found that there is no degradationin magnesium heat pipes. In further testing with more than 50 thermalcycles of a magnesium heat pipe with 0.6 wt % zirconium, no degradationor failure has been observed. Also note that the temperature for fluidswithin the vessel range from about room temperature (for ammonia) to upto 100° C. (for water).

[0011] It is anticipated that the process of the present invention wouldbe effective with aluminum as well. Note that for both aluminum andmagnesium systems, the addition of other “getter” alloying metals suchas titanium, hafnium and yttrium may also be included in order toincrease alloy strength and improve corrosion resistance. The alloys ofthe present invention are compatible with chemical acids, e.g., water,methanol and other alcohol fluids, as well as chemical bases such asammonia, pyridine, hydrazine, etc.

[0012] While the above description constitutes the preferred embodimentsof the present invention, it will be appreciated that the invention issusceptible to modification, variation and change without departing fromthe proper scope and fair meaning of the accompanying claims.

What is claimed is:
 1. In an apparatus including a magnesium alloyvessel substantially free of aluminum and zinc, said vessel having ahollow interior cavity containing a working fluid, the improvementcomprising: the formation of a stable, protective layer on the insidewall of the vessel, said protective layer establishing compatibilitywith the working fluid and preventing base metal corrosion by theworking fluid, wherein said vessel comprises magnesium in combinationwith an alloyed and/or dispersion strengthening, gettering metal.
 2. Theapparatus as recited in claim 1 wherein said stable protective layer isan oxide or nitride protective layer.
 3. The apparatus as recited inclaim 1 wherein said vessel is a heat pipe and/or a pumped-loop system.4. The apparatus as recited in claim 1 wherein said gettering metalcomprises from about 0.1-5 wt % of zirconium.
 5. The apparatus asrecited in claim 1 wherein the working fluid is ammonia.
 6. Theapparatus as recited in claim 1 wherein the working fluid is water. 7.The apparatus as recited in claim 1 wherein the gettering metal isselected from the group consisting of zirconium, titanium, hafnium andyttrium.
 8. The apparatus as recited in claim 1 wherein the getteringmetal comprises about 0.6 wt % zirconium alloy.
 9. The apparatus asrecited in claim 1, wherein said apparatus is a laptop computer.